Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. In general, the wireless communication system is a multiple access system capable of supporting communication with multiple users by sharing available radio resources. Examples of the radio resource include a time, a frequency, a code, transmit power, etc. Examples of the multiple access system include a time division multiple access (TDMA) system, a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, etc. The radio resource is a time in the TDMA system, a code in the CDMA system, and a subcarrier and a time in the OFDM system.
The wireless communication system is designed for the purpose of providing reliable communication to a plurality of users irrespective of their locations and mobility. However, a wireless channel has an non-ideal characteristic such as a fading phenomenon caused by a path loss, noise, and multipath, an inter-symbol interference (ISI), a Doppler effect caused by mobility of a user equipment, etc. Therefore, various techniques have been developed to overcome the non-ideal characteristic of the wireless channel and to increase reliability of wireless communication.
Adaptive modulation and coding (AMC) is a technique for increasing reliability of radio communication. The wireless communication system may use a channel quality indicator (CQI) to support the AMC. The CQI is information regarding a channel condition between a base station (BS) and a user equipment (UE). The BS determines a modulation and coding scheme (MCS) used in transmission by using the CQI received from the UE. If the channel condition is determined to be good by the use of the CQI, the BS may increase a data rate by increasing a modulation order or a coding rate. If the channel condition is determined to be poor by the use of the CQI, the BS may decrease the data rate by decreasing the modulation order or the coding rate. The decrease in the data rate may result in the decrease in a reception error rate.
The CQI may be periodically transmitted. The periodical transmission of the CQI implies that the CQI is transmitted without an additional request from the BS according to a period determined by the BS or according to a predetermined period. In case of the periodical transmission, a CQI information amount, a modulation scheme, a channel coding scheme, etc., may be predetermined. In this case, an overhead of signaling required for CQI transmission can be reduced. Meanwhile, a CQI information amount to be transmitted may be greater than a predetermined CQI information amount. In addition, since the wireless communication system is a time variant system, the channel condition changes over time. If a CQI transmission period is significantly long, the BS cannot recognize the changed channel condition. In this case, the BS cannot determine an MCS suitable for the channel condition. This may result in deterioration of reliability of the wireless communication system, and deterioration of overall system performance.
Therefore, to increase flexibility of CQI transmission, the CQI needs to be transmitted not only periodically but also aperiodically. The aperiodic transmission of the CQI implies that the CQI is transmitted in response to a request from the BS. In this case, a detailed transmission method (i.e., a radio resource, a modulation scheme, etc.) for aperiodic CQI transmission is problematic. This is because, if the transmission method is predetermined as in the periodic CQI transmission, flexibility of communication is decreased, and a signaling overhead is increased when signaling is performed on the radio resource, the modulation scheme, etc., at every aperiodic CQI request. Accordingly, there is a need to provide a method of effectively transmitting the CQI.